In The Clutch—Fast Talk With Jeff Smith

A good friend—Chris Douglas from Comp Cams—suggested a few weeks ago that I try something different with my column. He offered that since I’m constantly digging up fun technical stuff that I expand some of those ideas and perhaps dispel a few myths along the way. Chris’ idea appealed to me, so at least for the near future, we’ll take a look at some common myths, misconceptions, and folklore falsehoods in the automotive performance realm.

The first one I thought we’d get into was something I recently ran across on the internet. Let me first say that my intention is not to make anybody look bad. The internet contributor I will refer to was trying to help other hot rodders by showing how he accomplished an LS clutch conversion. Unfortunately, he committed a couple of minor errors in his commentary and if not corrected, the mistakes often perpetuate. Now that we’ve got the disclaimer out of the way, we can get into the techy stuff.

Our video host showed how he added a Muncie four-speed behind an LS engine. In the video he mentions that a stock LS 4.8L motor flywheel will work and the pressure plate bolt pattern is the same as earlier pressure plates – which is true.
It’s a mechanical necessity that the pressure plate must be aligned properly to the centerline of the flywheel/crankshaft to ensure no vibration. Early cars used special countersunk pressure plate mounting holes in the flywheel. These mounting holes were designed to accept a bolt with a matching dowel just underneath the bolt head that fit into these countersink holes. These bolts aligned the pressure plate to the flywheel.

Sometime roughly in the 1990s GM and all of the OE manufacturers changed the way they align the pressure plate to the flywheel. The new process uses three small dowel pins that align to matching holes in the pressure plate. This revised system also uses normal metric fasteners without the countersink.

The new metric fasteners for the GM pressure plate are 10mm x 1.50 thread size, which is roughly equivalent to a SAE 7/16-inch bolt. The earlier pressure plate bolts for small- and big-blocks were 3/8-inch x 16. The video swapper was using a stock truck LS flywheel but substituted an earlier pressure plate on this metric flywheel. However, when the metric bolts required by the late model flywheel would not fit through the 3/8-inch pressure plate mounting holes, he drilled them out to fit the larger metric bolts. Can you see the problem?

This is a early GM pressure plate bolt that uses the underhead area to fit into the countersink drilled into the pressure plate. This is why you can’t use just any bolt on these clutch assemblies. They need to be special pressure plate bolts.

As soon as he did this, the odds that the pressure plate is perfectly aligned with the crankshaft are very close to zero. To be fair, it’s entirely possible that even though the pressure plate is now off center, the vibration might be minimal, but that’s not the point of this discussion. The point here is that it’s important that everyone know how the process works so you can make the right decisions.
So the right course of action would be to use the late model pressure plate along with the LS flywheel. There are plenty of stock replacement and/or performance pressure plates so you have plenty of options. But the story doesn’t end there.

I called Will Baty at Centerforce to ask him if the finger height of the stock LS pressure plate is the same as a typical small-block Chevy. The reason for this question is because a swapper will likely use an older, small-block Chevy manual trans bellhousing. While that bellhousing will bolt up to five of the six LS bellhousing bolts, the clutch assembly will be 0.400-inch farther away from the stock release bearing position because the LS flywheel flange is that same distance shorter than a small-block Chevy.
Baty says that the stock LS truck style flywheel and pressure plate assembly places the release fingers at the same height as its older small-block and big-block cousins. This means the shorter LS crankshaft flange moves the entire assembly forward by 0.400-inch. The solution however is relatively easy. Centerforce (and perhaps other clutch companies) offers a longer release bearing that makes up nearly all the difference. A standard release bearing (PN 43001) for the traditional Muncie measures 0.715-inch height while the taller Centerforce PN 1466 bearing is 1.020-inch – longer by 0.305-inch.

This longer release bearing repositions the release arm to transcribe the correct arc and produce the required travel to release the clutch. The combination of these two ideas might help you the next time you’re swapping a Muncie or even a TKO 5-speed behind an LS engine. It pays to know the details.

In the upcoming months, we’ll take on some other fun myths and fractured performance fairy tales from the performance industry. And if you’ve got an idea you’d like to see discussed, drop us an email and maybe we’ll take that dive down the rabbit hole. But you gotta promise to come along!

This shows the pin arrangement used on later, metric clutch assemblies. The pin (arrow) matches up to specific holes in the pressure plate that locate the plate on the flywheel. You cannot swap an older pressure plate to a metric flywheel unless the pressure plate is drilled and pinned with a perfect centerline.

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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